More accurate observational determinations of masses, radii,
temperatures and abundances are needed in order to
improve the mass-luminosity relation at the end of the Main Sequence.
Those physical quantities are poorly defined in that mass range where
only a few stars have astrophysical parameters calculated with the
required accuracy.
New good-quality light curves of some late-type binary systems
have been obtained during a 6 year photometric
observational uvby and 
monitoring program of low mass
eclipsing binaries (Paper I and Paper II).
In this framework, particular attention was paid to BH Vir, for which
new radial velocity curve has been recently obtained (Popper
1995).
Photometric studies of the interesting eclipsing binary BH Vir have been carried out by several authors (Kitamura et al. 1957; Koch 1967; Sadik 1978; Botsula 1978; Hoffmann 1982; Scaltriti et al. 1985). More recently in a detailed analysis of the available light curves using the Wilson-Devinney code, Zhai et al. (1990) derived the relevant astrophysical parameters of both components. Vincent (1993), using surface imaging techniques includes BH Vir in the list of binary systems undergoing total eclipse.
The analysis of the photometric light curves for BH Vir is a difficult task mainly due to the large and very fast amplitude variations outside eclipses. Koch (1967), reported changes in the depth of the primary eclipse of a tenth of a magnitude, in only few days. The most generally accepted hypothesis to explain this light curve modulation outside eclipse is that the coldest star (or sometimes both) displays surface activity phenomena similar to the Sun, but of greater magnitude. The large body of existing data demonstrates that solar activity is a general phenomenon in stars of spectral type later than A. Rotation and convection are the basic ingredients (Hartmann & Noyes 1987).
Despite of showing intrinsic variability, BH Vir has been included in our program due to the lack of late-type eclipsing binaries without photometric variations outside eclipse. Relevant photometric elements can be extracted for such systems too, once the light curves have been cleaned of the activity waves attributed to the presence of spots in the stellar surface. We have implemented a new method to clean the light curves of the modulation due to activity. This method combines the determination of the activity wave with the search for the binary solution in an iterative process.
In this paper we present the detail analysis of BH Vir, based on the
good-quality uvby and light curves which has
been fully covered at least three times in four different epochs (Paper II).